The present invention relates to a battery charger capable of indicating charging condition of a secondary battery, and a method of judging its charging condition.
Generally, a residual electricity (capacity) checker for a primary battery such as an alkali dry cell and a manganese dry cell is a type where voltages of such battery are segmented into plural steps by predetermined threshold voltages and residual electricity is indicated in accordance with the steps. Such configuration becomes possible because a primary battery has the characteristic in which higher the voltage of the battery is, larger the residual electricity becomes.
However, in a secondary battery such as a nickel metal hydride battery (Ni/MH battery) or a nickel cadmium battery (Ni/Cd battery), since the open loop voltage remains substantially unchanged when it is fully charged or it is close to the fully charged condition except there is essentially no residual electricity, measurements of the battery voltage do not present any means to know how much is the residual electricity of the battery. Additionally, even in secondary batteries having the same electrical capacity, battery voltages may change depending on temperature, frequency of degradation (aging) and activation condition, thereby making it very difficult to know the residual electricity by measuring the voltage.
Such phenomena of secondary batteries also apply to the closed loop voltage when the batteries are being charged. Accordingly, in order to detect (indicate) the progress of charging in nickel metal hydride batteries or nickel cadmium batteries, it was conventional to completely discharge the batteries before charging and calculate the ratio of the measured charging time with respect to the standard time required for fully charging. However, even in such method, it is impossible to accurately detect the progress of charging if the rated capacity of the battery to be charged may change.
Some of conventional battery chargers for nickel metal hydride batteries have capacity checkers. However, such capacity checkers indicate whether the batteries are in the completely discharged condition or in the fully charged condition and thus fail to indicate the battery capacity in steps.